Rubber compositions used for a clinch, a chafer and a sidewall each of which comes into direct contact with a rim or curb are required to have excellent abrasion resistance and crack growth resistance. These rubber compositions are also required to have excellent handling stability (hardness), fuel economy, and elongation at break. In order to fulfill these requirements, for example, a rubber composition for a clinch, a chafer or a sidewall is used which includes a rubber component composed of natural rubber (NR) and butadiene rubber (BR), and contains zinc oxide typically in an amount of 2.5 to 5 parts by mass for each 100 parts by mass of the rubber component.
However, zinc oxide is an inorganic material difficult to disperse, and its use leads to poor abrasion resistance. Therefore, it is desirable to reduce the amount of zinc oxide. In addition, zinc oxide contained in tire rubber compositions has been regarded as a problem in recent years from the viewpoint of environmental pollution, and accordingly it is desired to reduce the amount of zinc oxide. However, the reduction in the amount of zinc oxide tends to cause reduction in hardness and deterioration of handling stability. Therefore, it has been practically difficult to reduce the amount of zinc oxide.
Other various attempts have been made to improve the handling stability (hardness), fuel economy, elongation at break, and abrasion resistance. For example, Patent Document 1 proposes a method of using VCR and a modified BR in combination as BR, Patent Document 2 proposes a method of using Tackirol V200 (produced by Taoka Chemical Co., Ltd.) as a hybrid crosslinking coagent, and Patent Document 3 proposes a method of optimizing the amounts of sulfur, vulcanization accelerator, and zinc oxide.
However, the techniques described in Patent Documents 1 to 3 have room for improvement in terms of improving fuel economy, elongation at break, and abrasion resistance in good balance while maintaining good handling stability (hardness) and processability (extrusion processability).
Patent Document 1: JP 2006-63143 A
Patent Document 2: JP 2009-84534 A
Patent Document 3: JP 2008-24913 A